Lithium-ion (including Lithium-ion polymer) batteries are popular in mobile applications due to their high energy density and low self-discharge rate. They have relatively high internal resistance that increases with aging. For high voltage applications, multiple cells are coupled in series into a battery pack. As Lithium-ion batteries cannot be equalized during charging simply by overcharging unlike some other secondary batteries, elaborate cell balancing is essential for such multi-cell Lithium-ion battery systems to cope with cell-to-cell variations such as internal resistance, state-of-charge (SOC), and capacity/energy (C/E) mismatch.
FIG. 1 depicts a flying-capacitor charge shuttling scheme for a battery pack in which multiple battery cells B1, B2, and B3 are coupled in series. It employs a capacitor C to transfer charge from a high voltage cell to a low voltage cell via a differential power bus (Vbus, Vbus′). Switches S1, S2, S3, S4, S5, and S6 are provided to connect the batteries to the power bus, connecting only one cell at a time to the power bus. The ‘flying-capacitor’ C is connected directly across the differential power bus (Vbus, Vbus′). Connecting it to a high voltage cell via the power bus charges the capacitor C; subsequently connecting to a low voltage cell discharges it, thereby transferring energy from the high voltage cell to the low voltage cell, consequently equalizing them.
Such flying-capacitor charge shuttling is energy-efficient only when the voltage differences between cells are small because the energy transfer efficiency is the ratio of low cell voltage to high cell voltage. If the voltage differences grow, the energy efficiency of the flying-capacitor charge shuttling plummets. For example, if the high cell voltage is 4V and the low cell is 3V, the energy efficiency of the flying-capacitor charge shuttling is no more than 75%. The rest 25% of the energy is dissipated into heat, as the charge transfer to and from the flying-capacitor is resistive.